The Role of Stress in Disease
Because of the high stocking densities used in most aquaculture operations andthe scarcity of water supplies free of aquatic microorganisms, a normally func-tioning immune system is absolutely essential to the health and physiological balance of the fish being cultured. The crowded conditions may increase initial susceptibility to infections and facilitate the horizontal or fish to fish transmis-sion of pathogens when infectious disease outbreaks do occur.
Stress is defined as physical, chemical or biological factors that cause bodily reactions that may contribute to disease and death. Many potential fish disease pathogens are continually present in the water, soil, and air or in the fish them-selves and outbreaks of clinical disease usually occur only when the fish is under some form of stress.
Aquaculture practices that increase stress are:
1. High stocking densities and poor water quality
2. Injury during handling (e.g. chasing, netting, sorting and shipping)
3. Improper nutrition
4. Poor sanitation
Some managerial practices that may help prevent stress:
1. Water quality
• Do not exceed the carrying capacity of ponds and tanks.
• Regularly monitor water quality parameters.
• Prevent the accumulation of organic debris, nitrogenous wastes, carbon dioxide and hydrogen sulfide.
• Maintain the appropriate or optimal pH, alkalinity, temperature and sa-linity for the species.
2. Handling and transporting
• Exercise speed and gentleness when handling fish.
• Use knitted mesh nets rather than knotted nets or better still, use smaller mesh size nets rather than larger mesh to reduce scale loss or entangle-ment of fins and finrays during capture.
• Minimize the number of times that fish are handled or lifted from water and if possible use anesthetics to slightly sedate the fish.
• Harvest, handle and transport fish at the point of their life cycle when they are least susceptible to stress, and in warm areas handle fish only during cooler periods of the day and add ice to the transport water to decrease fish metabolism and increase oxygen solubility.
• Maintain high levels of oxygen as this is critical for the rapid recovery of fish from the struggle of capture and handling; for freshwater fish add salt at 0.3 to 1.0 percent in the transport water to minimize osmotic stress and bacterial infection.
• Feed high quality diets that will meet the specific nutritional require-ments of the species as different species have different levels of require-ments for fatty acids, amino acids etc. and feeds that do not meet these needs are simply metabolized leading to increase excretion of wastes instead of being used for growth.
• Use proper feeding rates and feeding schedules.
• Store feeds in a cool dry place to preserve nutrients and prevent the growth of toxin producing fungi.
• Quarantine all new fish.
• Make sure that water supplies are not contaminated from the source.
• Immediately remove all dead fish and dispose them properly to prevent the spread of diseases.
• Observe good sanitation processes by disinfecting containers, nets and equipment to minimize transmission of parasites and disease from one population to another.